The present invention relates to a plate feeder for a lithographic platesetter and, more particularly, to a device for removing protective foils from a stack of plates.
A lithographic platesetter (or platesetter, for short) serves to record images on lithographic plates (which usually serve for offset lithographic printing) according to digitally supplied data. In operation of a platesetter, a fresh plate is loaded onto a recording surface (usually a drum), exposed to a modulated beam of energy, then unloaded. For automatic operation, a platesetter may be equipped with a plate feeder, which serves to pick one plate, at a time, from a stack of fresh plates and to transfer it to a loading mechanism; the latter loads the plate onto the recording surface. In some platesetters, the plate feeder and the loading mechanism may be an integrated system. The pack lies in a generally horizontal or inclined orientation and each time, the top plate is picked up for feeding. For convenience, the stack of plates is simply the contents of a pack of plates, as packaged and shipped by their manufacturer. Such packaging includes interleaving sheets of paper, or of similar foil material, with the plates; that is, between any two adjacent plates there is a sheet of paper. The purpose of the foil is to protect the sensitive face of the plate from damages, such as scratches, that may be caused by rubbing against the back face of the adjacent sheet.
Obviously, the paper must not be loaded onto the platesetter and therefore it must be removed from the top plate prior to the plate being picked up for feeding. Now, picking of the plate is usually carried out by means of vacuum suction cups, which are attached to a movable beam. According to prior art, these suction cups sometimes also serve to remove the protective paper sheets and to carry them to a discarding station. Such a method has several disadvantages:
(i) Often, the paper is porous; the vacuum then reaches the underlying plate and may cause it to be picked with the paper, unless the vacuum level is carefully controlled; this encumbers the vacuum system. PA1 (ii) For porous paper, the vacuum system must have a high rate of air flow--which increases its cost; it is noted that recycled and recyclable paper, which is used ever more frequently for the purpose, is usually porous, since coating it makes its recycling environmentally unsafe. PA1 (iii) Sometimes more than one sheet are found between any pair of plates; if the sheets are not sufficiently porous, the vacuum cups pick only one sheet at a time; this prolongs the process and makes automatic operation rather difficult, since it is then required to sense the presence of additional sheets.
In some plate feeders of prior art, there is a dedicated vacuum suction system for removing the paper. This overcomes the first of the enumerated disadvantages, but leaves the other two; moreover such a foil removal device is relatively complex and thus--expensive. In yet other feeders of prior art a system of rotating brushes is employed to remove the protective foils and transfer them to a discarding station. Such a system is bulky and, therefore, disadvantageous.
A similar plate feeder may also be utilized in other machines, for example--in an offset printing press that is equipped to record an inking image onto a plate directly on the press and which is further equipped with an automatic plate loading system. The present invention may be applicable to such feeders as well.
There is thus a widely recognized need for, and it would be highly advantageous to have, an economical, compact and effective device, within a plate feeder for a platesetter, for removing protective foil from the top plate of a stack, prior to feeding it to the platesetter.